A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing

Snow surface and sea-ice energy budgets were measured near 87.5°N during the Arctic Summer Cloud Ocean Study (ASCOS), from August to early September 2008. Surface temperature indicated four distinct temperature regimes, characterized by varying cloud, thermodynamic and solar properties. An initial warm, melt-season regime was interrupted by a 3-day cold regime where temperatures dropped from near zero to ?7°C. Subsequently mean energy budget residuals remained small and near zero for 1 week until once again temperatures dropped rapidly and the energy budget residuals became negative. Energy budget transitions were dominated by the net radiative fluxes, largely controlled by the cloudiness. Variable heat, moisture and cloud distributions were associated with changing air-masses. Surface cloud radiative forcing, the net radiative effect of clouds on the surface relative to clear skies, is estimated. Shortwave cloud forcing ranged between ?50 W m^?2 and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m^?2, except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface. Both cold periods occurred under tenuous, or altogether absent, low-level clouds containing little liquid water, effectively reducing the cloud greenhouse effect. Freeze-up progression was enhanced by a combination of increasing solar zenith angles and surface albedo, while inhibited by a large, positive surface cloud forcing until a new air-mass with considerably less cloudiness advected over the experiment area.     

气溶胶对AVHRR通道反照率的影响

通过数值模拟,考察了大气气溶胶对AVHRR通道1,2和整个太阳光谱区的行星反照率和地面反照率的影响,并研究了气溶胶对行星反照率和地面反照率之间关系的影响.     

Assessing satellite-derived net surface radiative flux in the Arctic

卫星反演的地表辐射通量数据最近有了改善和扩展,但其在北极的数据准确性还没有很好地评估。本文作者对三套卫星反演的北极地表净辐射通量数据(卫星产品包括SRB,ISCCP和APP-x),与从北极不同地区采集的四个现场测量值进行了比较。比较结果表明:就整体偏差、均方根误差和相关性而言,ISCCP的净辐射通量比SRB和APP-x更接近现场观测;就偏差的日变化而言,三套卫星产品结果没有明显优劣;就偏差的年际变化而言,ISCCP的净辐射通量比SRB和APP-x更准确。     

Parameterization of the shortwave radiative properties of water clouds for use in GCMs

Summary The influence of the micro- and macrophysics of water clouds on the scattering and radiative properties of clouds is investigated using versatile cloud drop size distributions (DSDs) and Mie theory for single scattering and the delta-Eddington approximation for multiple scattering. A new parameterization scheme for the shortwave radiative properties of water clouds is presented. As for single- scattering properties, a new parameterization for cloud optical thickness () is proposed. This is based upon the seperation of the dependence of on the total number of DSDs, the cloud thickness, and the liquid water content, combined with equivalent radius. The cloud bulk radiative properties, i.e., the flux reflectance, transmittance, and absorptance, are uniquely fitted by a dimensionless parameter (X) or the optical thickness. The parameterization is compared with other schemes. Finally, the features and potential use of the scheme are discussed.With 5 Figures     

Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust

In May–June 2006, airborne and ground-based solar (0.3–2.2 μm) and thermal infrared (4–42 μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m⁻² per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m⁻² were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between −19 and +24 W m⁻² for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.     

Streamwise heat flux budget in the atmospheric surface layer

Atmospheric surface-layer measurements of terms in the equation for the streamwise heat flux confirm previous results in both laboratory and atmosphere that the temperature-pressure gradient correlation acts as a sink, approximately equal in magnitude to the production term. The measured viscous dissipation term is independent of stability and represents less than 10% of the production term over the range of experimental stability conditions. Models for the temperature-pressure gradient correlation are compared with the measurements.     

A method to calculate cloud-free long-wave irradiance at the surface based on radiative transfer modeling and temperature lapse rate estimates

We demonstrate a method to improve the performance of commonly used parameterizations to calculate the cloud-free down-welling long-wave radiation at the surface. The method uses a monthly climatology of the effective radiating temperature of the atmosphere instead of the instantaneous screen-level temperature. The climatology of the effective radiating temperature can be derived from pyrgeometer measurements and was incorporated into two commonly used schemes. We compared the calculated cloud-free down-welling long-wave irradiances to high-quality pyrgeometer measurements from four Swiss sites. The discrepancies between observations and modified schemes can be reduced by up to 35 %, resulting in a model uncertainty close to 5 W m^?2 which corresponds to the measurement uncertainty of pyrgeometers. Furthermore, we introduce a new long-wave model which is based on radiative transfer calculations in the 8–14- $\upmu $ m wavelength range. In the remaining long-wave spectrum, the radiation is calculated using the Planck function with the effective radiating temperature of the atmosphere. The performance of this new model is consistent with the modified parameterizations.     

Direct human influence of irrigation on atmospheric water vapour and climate

Human activity increases the atmospheric water vapour content in an indirect way through climate feedbacks. We conclude here that human activity also has a direct influence on the water vapour concentration through irrigation. In idealised simulations we estimate a global mean radiative forcing in the range of 0.03 to +0.1 Wm^–2 due to the increase in water vapour from irrigation. However, because the water cycle is embodied in the climate system, irrigation has a more complex influence on climate. We also simulate a change in the temperature vertical profile and a large surface cooling of up to 0.8 K over irrigated land areas. This is of opposite sign than expected from the radiative forcing alone, and this questions the applicability of the radiative forcing concept for such a climatic perturbation. Further, this study shows stronger links than previously recognised between climate change and freshwater scarcity which are environmental issues of paramount importance for the twenty first century.     

Surface fluxes of heat and water vapour from sites in the European Arctic

Summary  Measurements of the surface fluxes of heat and water vapour were taken at four sites across the European Arctic as part of the EU funded LAPP project. The sites cover a range of latitudinal, altitudinal and climatic conditions. The most northerly site is near Ny-?lesund, Svalbard, a polar semi-desert with continuous permafrost. A second permafrost site is a fen area in the Zackenberg valley, East Greenland. Finally two sites in northern Finland, Skalluvaara and Kaamanen are on the southern boundary of the region affected by permafrost. At all sites measurements were made of the turbulent fluxes of heat and water vapour using eddy correlation equipment for at least one active season. The net radiation totals for July and August are similar at all sites. At the sites with permafrost a substantial proportion (over 20%) of the net radiation goes into soil heat flux, to thaw the soil moisture in the top metre. Of the remaining energy just over half is used for evaporation. At the Finnish sites the vegetation is largely deciduous and this is seen in the record with higher evaporative ratios in July and August, after the vegetation becomes green. The Finnish sites tend to have higher surface resistance to evaporation; however, the evaporative demand is greater leading to slightly higher evaporation rates. The two Finnish sites have a similar seasonal pattern determined by the water table and seasonality of the vegetation. The two northern sites show a pattern that is determined primarily by the variation of water table only. It is concluded that the water balance through the active season is influenced primarily by the history of snow cover. The seasonality of the vegetation, the permafrost and the depth of water table are also important influences. Received November 1, 1999 Revised April 17, 2000     

Effects of the coupling process on shortwave radiative feedback during ENSO in FGOALS-g

气候模式对ENSO期间负的短波辐射反馈的模拟依然有挑战性,以往的研究主要侧重于大气或者海洋模式中具体物理过程的影响。为了探究耦合作用对其负反馈的效用,本研究分析对比了两个大气环流模式(GAMIL1和GAMIL2)以及相应的耦合模式(FGOALS-g1和FGOALS-g2)对ENSO的模拟,结果显示新旧两个版本的耦合作用对短波反馈的影响是不一样的,主要来源于云量反馈和动力学反馈的差异;通过对反馈的非线性过程的分析进一步揭示了新旧两个版本相反的耦合作用来自于它们对于厄尔尼诺和拉尼娜的不同相应。     

Direct shortwave radiative forcing of the Asian dust aerosol on dust emission

The physical processes of the feedback mechanism of direct shortwave radiative forcing of the Asian dust aerosol on dust emission has been examined using simulated results with the coupled (with dust shortwave radiative forcing) and the non-coupled model (without dust shortwave radiative forcing) based on the MM5 model and the Asian Dust Aerosol Model on 19 March 2002. The results indicate that a significant dust emission reduction occurs in the high dust concentration (HDC) region of the dust source region whereas an enhanced dust emission appears in the downstream of the dust source region. It is found that Asian dust aerosols raised during the daytime by the strong surface wind cause negative shortwave radiative flux near the surface, which in turn reduces the sensible heat flux causing the cooling of the air, thereby enhancing stable stratification. The dynamic adjustment of the negative radiative flux of the dust induces a positive pressure anomaly over the HDC region and a negative pressure anomaly toward the synoptic low pressure center, resulting in a dipole shape of pressure anomaly field near the surface. The associated secondary circulation of this pressure anomaly together with the reduction of turbulent intensity due to the reduced sensible heat flux reduces the low-level wind speed thereby reducing dust emission in the upstream of the HDC region of the dust source region (Region I), while enhancing the low-level wind speed in the downstream region (Region II), which in turn enhances dust emission. This enhanced dust emission is smaller than the emission reduction in the upstream, resulting in overall dust emission reduction during the daytime.     

Distribution,speciation, and budget of atmospheric mercury

Total gaseous mercury (TGM) concentrations over the Atlantic Ocean and over Central Europe were measured repeatedly in the years 1978–1981. The latitudinal TGM distribution showed a pronounced and reproducible interhemispherical difference with higher TGM concentrations in the Northern Hemisphere. TGM was found to be vertically well mixed within the troposphere. The TGM concentration seems to increase with time at a rate of 10±8%/yr in the Northern and 8±3%/yr in the Southern Hemisphere. Measurements of mercury speciation showed that elemental mercury is the main TGM component contributing more than 92% and 83% of TGM in marine and continental air, respectively. The tropospheric mercury burden was calculated to be 6×10⁹g. The interhemispheric distribution and temporal and spatial variability of TGM imply a tropospheric residence time of TGM of about 1 yr. Sink strengths calculated independently from the measured mercury concentration on particles and in rainwater are consistent with the above figures.     

Short-term temporal variability of atmospheric surface pressure and wind speed in the Canadian Arctic

In this study, the spatial differences and interannual fluctuations in temporal variability of surface pressure and wind speed on different timescales at 12 locations in the Canadian Arctic are documented. Temporal variability is defined as the mean-squared value of time tendencies smoothed by running means over different intervals. It is shown that variability on timescales of up to 1 month is itself highly variable, both in space and time. Due to the significant impacts from the immediate geographical environment, for surface wind speed, these variations show no spatial pattern on a continental scale, and only a few persistent trends over periods of more than 10 years. Also, spatial and temporal anomalies do not significantly depend on timescale. Contrary to this, spatial and temporal variations in the variability of surface pressure and its changes with time show well-defined regional similarities, as well as a strong spatial and temporal dependence on timescale. As a result, variability of surface pressure on timescales between 1 and 3 days increases from the northeast region of the domain towards the southwest. On longer timescales, this spatial gradient is reversed. The consistent spatial pattern across the study domain suggests that variability of surface pressure is primarily governed by large-scale atmospheric processes, and is to a large extent independent of the exact geographical setting.     

Reducing discrepancies in atmospheric heat budget of Tibetan Plateau by satellite-based estimates of radiative cooling and cloud-radiation feedback

Summary Past investigations of the summertime heat budget over the Tibetan Plateau have not included detailed estimates of radiative cooling (Q _R ) nor have they carefully considered the effects of cloudiness on this term. The various attempts to eyaluate different combinations of heat and moisture budget torms and to understand the sources of energy to the summer armospheric heat source over the plateau are not in agreement, partly because of remaining discrepancies in the radiative and turbulent flux components, and partly because until recently, the conventional data sets needed for independently estimating the total heating and moistening terms have been incomplete. The uncertainties in the radiative term have also led to difficulties in assessing the uncertainties in the other budget terms, since no study to date has assembled a complete enough data set to allow a unified calculation of all budget quantities or to obtain budget closure. Recently published results of Yanai and his colleagues involving apparent heat source calculations for the plateau region based on a much improved FGGE data set, have motivated the examination of whether more detailed radiative calculations can help resolve past discrepancies in the budget terms on a monthly time scale. This study uses a continuous time series of 22-km resolution INSAT geosynchronous satellite measurements and ECMWF profile analyses in conjunction with medium spectral resolution radiative transfer models to estimate the slicrtwave and longwave components of the radiative cooling term and the role of cloudiness on these components for the 1988 summer period. The calculations reveal both meridional and zonal structure in radiative divergence across the plateau associated with the substantial gradients of cloudiness and aridity that dominate the summertime plateau climatology. The calculations also indicate that the magnitudes of both cloud-induced shortwave heating and longwave cooling over the plateau are much greater than over low-elevation regions. Moreover, since cloud-induced longwave cooling exceeds cloud-induced shortwave heating, the bulk effect of clouds is to radiatively cool the plateau atmosphere. The high resolution calculations are reduced to monthly averaged budget quantities for analyzing whether existing discrepancies in the plateau heat budget can be resolved. Although there is no means to rigorously verify the accuracies and representativeness of the individual budget terms, the new radiative estimates combined with the most reliable current estimates of total heating and turbulent fluxes, produce near closure (within 4%) of the plateau heat budget for the June to August period.With 10 Figures     

两种对流参数化方案对辐射能量收支的影响研究

利用中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室的格点大气环流模式(GAMIL)1.0版设计了两组数值模拟实验来研究两种不同的对流参数化方案对辐射能量收支的影响.这两种对流参数化方案分别是:Zhang and McFarlance/Hack方案(简称ZM)和Tiedtke/Nordeng方案(简称 TN).对应的数值模拟实验分别取名为EX-ZM和EX-TN.通过对实验结果的分析表明:在对流过程中,EX-ZM允许深对流和浅对流同时发生,因此两种对流同时在模式低层消耗了更多的水汽,释放了更多的潜热,引起了更大的增温;EX_TN每次只允许一种对流发生,也就避免了不同类型的对流在同一层同时消耗水汽的现象.因此对流过后,EX-ZM的环境空气相对湿度较小,而EX-TN周围空气的相对湿度较大,有利于低云云量的生成和大尺度的凝结,因此EX-TN模拟的低云云量偏多,低层的云水含量偏高,模式低层的云光学厚度偏大,这就使得EX_TN中更多的太阳短波辐射通量被云反射掉,严重低估了模式对短波波段的辐射通量的模拟.此外,不同的对流参数化方案通过改变云的长波发射率和降水,进而影响了模式对长波波段的辐射通量、感热和潜热通量的模拟.